Seed plants were the most recent evolutionary step


The big picture: We now live in a seed plant world
- Seeds changed the course of plant evolution
- Gymnosperms evolve ~360 mya
- climate became drier = selection pressures?
- Elkensia first fossil seed plant
- Seeds consist of embryo & nutrients
- surrounded by protective coat
- seeds can be dispersed long distances
- less dependence on water
- 2 living seed lineages
- gymnosperms (naked seed)
- angiosperms (covered seed = ovary + fruit)


Why seeds matter so much…
- Before seeds, spore was only protective structure
- allowed dispersal but…
- embryo still exposed
- Seeds can remain dormant for a long time
- Gymnosperms: seeds exposed on sporophylls, form cones
- Angiosperms: seeds enclosed inside ovary

Seed plants further adapt to life on land
- Reduction of gametophyte
- mosses → ferns → seed plants
- gametophyte became microscopic
- Heterosporous life cycle
- 2 kinds of spores
- form male/female gametopytes
- Ovules
- sporangium kept inside sporophyte
- ovule contains female spores
- Pollen
- male spores develops into pollen grain




4 living lineages of gymnosperms
- Cycads: 300 living species
- earliest evidence of insect pollination
- Cycads and dinosaurs lived together
- Gnteophytes: small and super weird group
- Ginkos: Ginkgo biloba is only species left!
- Conifers: ~600 species (cones)
- evergreen and deciduous
- leaves are needles or scales

Welwitchia: the weirdest f’in plant on Earth

Gymnosperm reproduction: basics
- Male and female cones on same plant
- sporophyll → sporangia → spores
- scales → modified leaves → sporophyll
- pollen (male) and ovulate (female) cones
- Male cones house microsporangium
- microsporangium produces microspores
- microspores develop into pollen grains
- male gametophyte inside pollen grain
- pollen dispersed by wind
- male cones usually on bottom of tree

Gymnosperm reproduction: basics
- Female cones have 2 ovules per scale
- each ovule has megasproangium
- megasporangium produce 4 megaspores
- 1 megaspore survives
- megaspore develops gametophyte with egg
- Pollen grain reaches ovule and germinates
- pollen tube grows toward megaspore
- sperm → pollen tube → egg
- seed forms, dispersed by wind


Flowering plant diversity
- Most diverse living group of plants
- 14,000 genera, 350,000 species
90% of plant diversity
- 2 classes: Moncots and Dicots
- monocots = one seed leaf in embryo
- dicots = two seed leaf in embryo

Angiosperm evolution: plants and insects


Angiosperm reproduction: Flowers
- Flower is a modified stem for reproduction
- with 4 types of modified leaves
- Sepals: leaves at base which enclose flower
- Petals: colored leaves to attract pollinators
- less colored if wind pollinated
- Two types of fertile organs to make spores
- sporophylls (male and female)

Flowers - Male
- Stamens: microsporophylls
- produce microspores that develop into pollen
- Anther: where microsporangia develop
- microspores produced
- microspores → pollen
- pollen → male gametophyte
- Filament: stalk that holds the anther

Flowers - Female

- Style: leads from stigma to ovary
- Ovary: contains ovules
- megasporangium inside ovule produce 4 megaspores
- 1 megaspore survives to make female gametophyte
Angiosperm reproduction: double fertilization

Comparing seed plant reproduction
